Weight is often a factor in the design of semiconductor devices, particularly when those devices are to be employed in aerospace applications. Weight is a very significant parameter for large area semiconductor devices such as photovoltaic generating units. Photovoltaic devices based upon single crystal materials are inherently rigid, brittle and heavy, and these factors often limit their applicability. A number of thin film semiconductor materials have been developed which are equivalent, and in many instances superior, to single crystal materials. These thin film semiconductor materials are light in weight, and amenable to being deposited in relatively thin, flexible films over large areas and at relatively low cost. These thin film materials include amorphous, nanocrystalline, microcrystalline and polycrystalline materials as well as composites.
Thin film semiconductor materials are usually supported on a substrate, and such substrates should be compatible with the semiconductor materials and have good dimensional stability under conditions encountered during the fabrication of the device and its service life. In many instances, thin film semiconductor materials are fabricated by vacuum deposition techniques such as glow discharge deposition, evaporation, sputtering or chemical vapor deposition. As a consequence, the substrate will be exposed to relatively high temperatures and low atmospheric pressures. In addition, thin film semiconductor materials, particularly for large scale applications, are fabricated by automated, high volume, roll-to-roll deposition processes, and in such processes the substrates are additionally subject to relatively high tensile forces.
It will thus be appreciated that substrates for thin film semiconductor materials must meet a number of relatively high standards for strength and stability. In many instances, thin film semiconductor materials are fabricated upon rigid glass or ceramic substrates. Such materials are heavy, relatively expensive, and not readily amenable to roll-to-roll deposition processes, as is required for the fabrication of large area devices. In many other instances, thin film semiconductor devices are fabricated upon flexible, metallic substrates. While devices thus fabricated are flexible and relatively light in weight, the metal substrate comprises almost all of the bulk of the device, and for large area, aerospace applications, it is very desirable to further limit the weight of the devices. As a consequence, a number of efforts have been made to fabricate large area semiconductor devices onto very thin polymeric substrates. A number of polymeric materials, particularly polyimides, and equivalent materials, have very good dimensional stability and are compatible with thin film semiconductor materials. However, such polymeric materials tend to soften under the high temperature conditions encountered in many vacuum deposition processes, and hence do not have sufficient tensile strength, at elevated temperatures, to be used in high volume fabrication processes such as roll-to-roll processes. In addition, relatively thin substrate materials can be difficult to handle during other stages of device fabrication such as encapsulation, connection of conductive leads, module fabrication and the like.
A number of approaches have been implemented in the prior art for the problem of fabricating semiconductor devices on lightweight substrates. As disclosed in U.S. Pat. Nos. 4,663,828 and 4,663,829, lightweight photovoltaic cells are fabricated by a process wherein semiconductor material is deposited onto a surrogate substrate. The layers of semiconductor material are then removed from the surrogate substrate by thermally shocking the semiconductor material to cause it to debond from the substrate. The freed semiconductor film is then encapsulated or otherwise configured into a photovoltaic device. Since the relatively thick surrogate substrate functions as a heat sink, relatively large thermal shocks must be applied to the structure to effect the delamination. Also, the freed semiconductor film is relatively fragile and can be subject to damage. Accordingly, it is preferable that the semiconductor device remain upon the substrate upon which it was initially deposited.
In other approaches to this problem, as noted in the aforementioned patents, semiconductor devices are fabricated upon metallic substrates which are then either completely removed or thinned down by an etching process. In other variations of this technique, semiconductor devices are fabricated upon relatively thin polymeric substrates which in turn are supported upon metallic substrates which are chemically etched away. In both of these approaches, relatively large amounts of metal must be removed in chemical baths which are highly corrosive. In addition to being time consuming and expensive, these techniques can damage semiconductor layers, contacts and other portions of the semiconductor device. Therefore, there is still a need for a method for the fabrication of semiconductor devices upon relatively thin substrates, which method does not require chemical etching of a substrate and which does not expose semiconductor layers to high thermal stresses.
As will explained in greater detail hereinbelow, the present invention provides a simple, easy to implement method wherein a thin polymeric substrate is supported upon a relatively thick support member during fabrication and processing of a semiconductor device. In accord with the present invention, the semiconductor device is then stripped from the support member by a skiving process implemented through the use of a beam of radiant energy. The process does not directly expose the semiconductor layers to any source of thermal energy or to any corrosive environment. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.